The invention relates generally to movement of a bubble of one fluid in another fluid within a capillary, and more particularly to switches for optical signals, to fluid valves, to fluid pumps, and to liquid degassers.
Bubbles in capillaries are useful in several respects. In optical devices they possess a different index of refraction than a surrounding liquid and so can reflect or refract optical signals. In addition they can be moved about within capillaries by exploiting temperature-dependent surface tension effects which tend to force bubbles from colder regions to warmer regions, by exploiting geometric effects which tend to squeeze bubbles out of small capillaries into larger capillaries, and by exploiting wetting effects which tend to favor the presence of a first fluid which easily wets a capillary wall over a bubble of a second fluid which does not wet that same wall as well.
Capillary devices that exploit the change in surface tension of a bubble with temperature are said to work by thermocapillarity, also known as the Marangoni effect (see, for example, L. E. Scriven and C. V. Sterling, xe2x80x9cThe Marangoni Effects,xe2x80x9d Nature, V 187, p 186 (1960)). An example of recent work published on an optical switching device that exploits thermocapillarity is Makoto Sato, et al, xe2x80x9cWaveguide Optical Switch for 8:1 Standby System of Optical Line Terminals,xe2x80x9d paper WM16, Technical Digest, OFC ""98 Optical Fiber Communication Conference and Exhibit, Feb. 22-27, 1998, San Jose Convention Center, San Jose, Calif., pp 194,195. The Marangoni effect is also exploited to control bubble movement for optical switching in the invention xe2x80x9cTOTAL INTERNAL REFLECTION OPTICAL SWITCHES EMPLOYING THERMAL ACTIVATION,xe2x80x9d U.S. Pat. No. 5,699,462. In that patent, for example, the embodiment showed in FIG. 29 therein, in a capillary 304 with a tapered width, a bubble is generated at the resistor 308, and is held in place at that resistor by the Marangoni effect which counteracts the geometry-induced force on the bubble due to the tapered width of the capillary. When heating of the resistor is terminated, the bubble moves from resistor 308 toward the top left of the figure along tapered capillary 304, where movement is generated by geometric forces. Thus the operation of the device in FIG. 29 of that patent depends on an opposing balance between Marangoni forces and geometry-induced forces.
It will be understood from the description of the present invention that forces due to geometric effects can be much larger than forces due to the Marangoni effect. Because of the relative feebleness of the Marangoni effect, the geometric effect designed into the device of FIG. 29 of U.S. Pat. No. 5,699,462 to balance the Marangoni effect must also be feeble and the operation of that device is relatively slow and susceptible to interference from mechanical shock. There still exists a need for a microfluidic device which uses large-magnitude forces to provide rapid and stable optical switching, and the present invention meets that need.
Bubbles are also usefull as valve elements to control fluid flow in capillaries. A bubble can be made to reside at a given position in a capillary by some combination of temperature effects and geometric effects which creates a local energy potential minimum for the bubble, and by blocking or nearly blocking the capillary can then impede the flow of a surrounding fluid. See, for example, John Evans, et al, xe2x80x9cPlanar Laminar Mixer,xe2x80x9d Proceedings of the Tenth annual International Workshop on Micro Electro Mechanical Systems, Nagoya, Japan, Jan. 26-30, 1997, IEEE Catalog Number 97CH36021, pp 96-101. However, the Evans et al. paper teaches no technique for removing the bubble from its local energy potential minimum. Devices that do not provide adequate bubble removal can suffer from xe2x80x9cvapor lockxe2x80x9d if the bubble is composed of gas liberated from dissolved gas in the liquid, because such a bubble will fail to disappear when the heating resistor which creates it is turned off. There still exists a need for a reliable technique for confining a bubble in a channel securely, and when desired, for releasing and removing the bubble from the channel, in a repeatable and efficient manner. As will be seen in detail below, the present invention teaches such a technique.
Bubbles are also useful as pumping elements in capillaries. An expanding bubble in a capillary or chamber can act as a piston, displacing the surrounding fluid and causing it to move in a direction dictated by the capillary geometry. The same work by John Evans, et al, xe2x80x9cPlanar Laminar Mixer,xe2x80x9d referenced above, employs an alternately expanding and contracting bubble in a chamber as a piston element. Like the valve described in the same work, the piston bubble can suffer from xe2x80x9cvapor lock.xe2x80x9d There still exists a need for a pump employing a bubble piston which removes that bubble from the pumping chamber when desired, and the present invention teaches such a pump.
The present invention uses the difference in energy potential associated with a bubble in different regions of a capillary to trap the bubble within a region of the capillary when such trapping is desired, and does so while providing resistance to perturbations, which could lead to undesired changes in state. The difference in energy potential can be geometry-dependent and/or materials-dependent. The present invention also uses difference in energy potential to remove the bubble from the trap when desired, and does so more rapidly than prior art devices. The invention is useful for optical switching, for fluid valving, for fluid pumping, and for liquid degassing.
Operation of the device is as follows. In a capillary containing a first fluid, a wall-confined bubble of a second fluid is introduced into a designed-in trap in the capillary by some means, for example by boiling a liquid constituting the first fluid using a heating resistor to create a bubble of a vapor which constitutes the second fluid. Bubbles can also be introduced using electrical, chemical, electrolytic, pneumatic, hydraulic, optical, inertial, ultrasonic, and microfluidic techniques, including injecting bubbles from a source (e.g., a gas bubble from a gas source). Introduction of the bubble accomplishes a first desired event, such as switching of an optical beam or blocking of a fluid channel. The bubble is trapped because it sits at a local energy potential minimum within the capillary, and moving it would require an input of energy.
Next, the energy of the bubble is increased by, for example, increasing the power input to a heating resistor to introduce more gas, thereby increasing the size of the bubble. As the bubble grows its energy increases, and as it grows it encounters a designed-in spatial asymmetry (geometrical and/or material) in the energy potential of the capillary adjacent to the trap. It tends to grow in the direction of least energy potential. Growth of the bubble can accomplish a second desired event, such as pumping of the volume of the first fluid displaced by the bubble during its growth.
Then, the growing bubble reaches a metastable energy maximum and it encounters a designed-in region of low spatial energy potential within the capillary. Further growth of the bubble causes it to intrude into the region of low spatial energy potential, and the bubble becomes positionally unstable. In a manner analogous to the siphoning of water through a hose from a hillside pond to a lower pond due to gravitational energy potentials, the bubble moves from the trap and flows rapidly into the region of low spatial energy potential. As it moves, it accomplishes a third desired event, such as switching of an optical beam or unblocking of a fluid channel.
In some embodiments, as the bubble leaves the trap in a direction defined as downstream, it pushes downstream some of the first fluid ahead of it and sucks a volume of the first fluid from a direction defined as upstream to refill the volume of the trap.
Because the invention employs designed-in asymmetries in energy potential to accomplish bubble trapping and bubble movement, it is referred to as an asymmetric bubble chamber, or xe2x80x9cABC.xe2x80x9d
The following terminology is used below to refer to the various spatial regions of the ABC.
The trap volume in which the bubble is initially trapped is called the xe2x80x9cgate.xe2x80x9d The region of low spatial energy potential into which the bubble exits from the gate is called the xe2x80x9cdrain.xe2x80x9d The region of relatively higher energy potential between the gate and the drain constitutes a potential barrier and is called the xe2x80x9cbarrier.xe2x80x9d The region from which the first fluid enters to refill the gate after the bubble has exited from the gate to the drain is called the xe2x80x9csource.xe2x80x9d In the usual mode of operation of the device in which the first fluid enters from the source, the source is upstream of the gate, the gate is upstream of the barrier, and the barrier is upstream of the drain, where xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d refer to the usual direction of flow of the first fluid.
The source has an energy potential for a wall-confined bubble of a second fluid in the first fluid called the xe2x80x9csource potential.xe2x80x9d However, in some embodiments, provided that fluid can refill the gate region, the source may be absent or plugged. For example, a device can have two barriers and one of them can act as the source.
The gate has an energy potential for a wall-confined bubble of a second fluid in the first fluid called the xe2x80x9cgate potentialxe2x80x9d which is less than the source potential.
The barrier has an energy potential for a wall-confined bubble of a second fluid in the first fluid called the xe2x80x9cbarrier potentialxe2x80x9d which is higher than the gate potential but lower than the source potential.
The drain has an energy potential for a wall-confined bubble of a second fluid in the first fluid called the xe2x80x9cdrain potentialxe2x80x9d which is less than the gate potential.
From the above description of relationships among the four energy potential regions it will be appreciated that the gate has a region of local energy potential minimum called a xe2x80x9cpotential well.xe2x80x9d
Prior-art microfluidic switches employing a bubble in a capillary tended to be unstable and the speed of operation tended to be slow. In the present invention a bubble is initially trapped in a potential well, but then can overcome a substantial energy barrier and rapidly move outside the well. Thus, a system with a switch, valve, or pump of the present invention is stable and is resistant to unintended perturbation by vibration or temperature, but can also operate rapidly.
It will be appreciated that the invention can be operated in cycles, where the speed of operation is limited first by the speed at which a bubble can be introduced into the gate, and second by the speed at which a bubble can be moved from the gate to the drain.
Moreover, this operation need not be accomplished by the mechanical motion of solid parts. Rather, the energy potentials of the source, gate, barrier, and drain are defined by the geometry of the channel, or by the materials composition of the channel plus the first fluid plus the second fluid, or by a combination of geometry and materials composition. The invention allows bubbles to be introduced and moved entirely by fluidic means which, for example, can be controlled thermally by electrical heating.
In one embodiment of the present invention a capillary is filled with a liquid such as water and the walls of the capillary are a hydrophilic material such as glass. A steam bubble is introduced into the gate by using a small electrical resistor to boil the water. The bubble is large enough to substantially fill the gate, and its introduction causes a first switching event. If the capillary is part of an optical crosspoint switch such as that described in U.S. Pat. No. 5,699,462, the presence of the bubble causes a first optical switching event. If the capillary is part of a fluid flow system, the presence of the bubble acts to block fluid flow within the capillary and so causes a first fluidic switching event. In either case the local energy potential minimum within the gate acts to trap the bubble, even in the presence of disturbances such as mechanical acceleration or liquid pressure fluctuations. In such a hydrophilic setting the gate is a chamber of larger width than the adjacent source and barrier portions of the capillary.
In a second switching event for the same embodiment of the invention, which returns the system to its initial state, the volume of the bubble is increased, by increasing the heating power to the resistor, so that it fills the volume of the gate plus the volume of the barrier and begins to intrude into the volume of the drain. The enlarged bubble is then siphoned by surface tension forces from the gate, through an adjacent narrowing of the capillary (which is the barrier in a hydrophilic setting), and into an even wider portion of the capillary (which comprises the drain in a hydrophilic setting). At the same time, liquid refills the gate from upstream through a narrow portion of the capillary (which is the source in a hydrophilic setting).